Polylactic acid is practically a melt-moldable macromolecule. Since the polylactic acid is biodegradable, it has been developed as a biodegradable plastic, which is degraded in a natural environment after use, and is released as carbon dioxide gas and water. Meanwhile, renewable resources (biomass) derived from carbon dioxide and water are used as raw materials of the polylactic acid itself, and thus carbon dioxide released after use does not contribute to increase an amount of carbon dioxide in the earth environment. As such, in recent years, the polylactic acid is expected to be used as an eco-friendly material because of its property of carbon neutrality. Moreover, lactic acid, which is a monomer of polylactic acid, can be manufactured at low cost by using a fermentation method utilizing microorganisms, and is expected to be used as an alternative material for a universal polymer made from a petroleum plastic. However, compared to petroleum plastics, polylactic acid is less productive because it is not much heat resistant and not much durable, and its crystallization rate is low. Thus, its use has been largely limited.
To compensate for the disadvantage of polylactic acid, a polylactic acid stereocomplex is expected to be used. The polylactic acid stereocomplex can be formed by mixing optically-active poly-L-lactic acid and optically-active poly-D-lactic acid. The melting point of a polylactic acid stereocomplex is 220° C., that is, the melting point is 50° C. higher than that of a polylactic acid homopolymer, which is 170° C. The polylactic acid stereocomplex is usually formed by mixing a poly-L-lactic acid solution and a poly-D-lactic acid solution, or by heat melt mixing poly-L-lactic acid and poly-D-lactic acid. However, if high molecular weight poly-L-lactic acid and high molecular weight poly-D-lactic acid are heat melt mixed, a material that is heat resistant as well as durable is difficult to be obtained because many melting point peaks derived from polylactic acid homopolymers exist even if the ratio of mixing composition is 50:50.
On the other hand, a polylactic acid block copolymer is gathering attention as a novel method of forming polylactic acid stereocomplexes. Poly-L-lactic acid segments containing L-lactic acid as a main component and poly-D-lactic acid segments containing D-lactic acid as a main component are covalent bonded to form the polylactic acid block copolymer. Although the molecular weight of the polylactic acid block copolymer is high, it well forms a stereocomplex crystalline form, and the melting point derived from stereocomplex crystals can be observed. Thus, a material, which has excellent heat resistance, crystallization properties, mechanical properties, and durability, can be obtained. Accordingly, application of the polylactic acid block copolymer to fibers, films, and resin molded articles having high melting points and excellent crystallinity has been attempted.
In the molding process of polylactic acid, the polylactic acid is heat melted at the temperature of the same as or above the melting point of the polymer, and is molded in a desired shape. However, if melting retention is conducted at a high temperature, thermal degradation may occur. The residual of a metal catalyst used to polymerize polylactic acid is the main cause of thermal degradation, and the residual of the metal catalyst facilitates a reaction for removing lactide from the end of polylactic acid, which results in decrease of a molecular weight or a gross weight. If the weight is decreased at the time of molding, the physical characteristic of the molded article is affected, and thus a catalyst deactivating agent such as a metal phosphate is added in the polymer after polymerization to reduce activity of a tin catalyst so that thermal degradation is decreased, and thermal stability is increased. As mentioned, regarding polylactic acid and polylactic acid stereocomplexes, polylactic acid resin compositions have been widely developed to improve the characteristics of molded articles (see, for example, Japanese Patent Application Laid-open No. 2003-192884, WO 2012/029393, WO 2009/119336, Japanese Patent Application Laid-open No. 2010-84266 and WO 2012/111587).
In JP '884, a polylactic acid polymer composition, in which a metal phosphate ester is added as a nuclear agent to a polylactic acid stereocomplex including poly-L-lactic acid and poly-D-lactic acid, is disclosed. The metal phosphate ester is an aromatic metal organophosphate ester containing an alkali metal atom or an alkaline earth metal atom. A polylactic acid polymer composition containing the aromatic metal organophosphate ester has a higher cooling crystallization temperature and an excellent crystallization characteristic, and thus good molding in a metal mold can be expected. However, although a melting point and a crystallization characteristic are improved by using the polylactic acid stereocomplex, the metal organophosphate ester used as the resin composition is less effective in deactivating a tin compound contained in the polylactic acid. Thus, thermal degradation occurs at the time of heating, and the weight is decreased. As a result, mechanical properties after molding may be reduced, and durability may be affected.
In WO '393, a polylactic acid block copolymer including poly-L-lactic acid segments containing L-lactic acid as a main component and poly-D-lactic acid segments containing D-lactic acid as a main component is disclosed. In this art, since poly-L-lactic acid segments containing L-lactic acid as a main component and poly-D-lactic acid segments containing D-lactic acid as a main component are covalent bonded, stereocomplex crystals can be formed even if the molecular weight of the polylactic acid block copolymer is high, and the heat resistance and the crystallization properties of the polylactic acid block copolymer are excellent compared to those of homopolylactic acid. The compound used as a catalyst deactivating agent in this art is a phosphate compound or a phosphite compound. If these compounds are contained in the polylactic acid block copolymer, thermal degradation upon heating is suppressed, and thermal stability is increased. However, the problem is that, since a cooling crystallization temperature is lowered and enthalpy of crystallization is decreased, the crystallization properties are also decreased.
On the other hand, to improve the hydrolysis resistance, suppress the thermal degradation, and improve the thermal stability of a polylactic acid resin, the art in which an alkali metal phosphate represented by sodium dihydrogen phosphate is mixed is disclosed (WO '336, JP '266 and WO '587). This is different from an organic phosphoric acid compound used in JP '884 and WO '393.
In WO '336, the hydrolysis resistance of a polylactic acid resin is improved by adding an alkali metal phosphate to a polylactic acid resin. In this art, since the alkali metal phosphate captures a hydrogen ion derived from a carboxy group existed on the end of polylactic acid, a buffer effect prevents pH to be varied even if some hydrogen ions are generated from the carboxy group existed on the end of polylactic acid, and thereby improves a hydrolysis resistance.
In JP '266, a dihydrogenphosphate of an alkali metal and a carboxy group reactive end-capping agent are mixed in a polylactic acid resin to improve a hydrolysis resistance. Similarly to WO '336, the dihydrogenphosphate of an alkali metal used in this art captures a hydrogen ion released from an end carboxy group so that the hydrolysis resistance is improved. Moreover, in JP '266, since the carboxyl end-capping agent reacts with the carboxy group of polylactic acid, the hydrolysis resistance is further improved.
In WO '587, a thermoplastic resin composition, in which sodium dihydrogen phosphate is mixed in a resin composition including a styrene resin, a graft copolymer, and an aliphatic polyester represented by a polylactic acid resin, is disclosed. In the thermoplastic resin composition disclosed in WO '587, a styrene resin represented by an ABS resin and a graft copolymer are mixed in the polylactic acid resin. Accordingly, the impact resistance of the polylactic acid resin, which is a problem to be solved, is improved. Moreover, by containing phosphoric acid and/or sodium dihydrogen phosphate, alkalinolysis of an aliphatic polyester resin is prevented to improve thermal stability, as well as to suppress irritating smell generated at the time of molding.
As for WO '336, JP '266 and WO '587, although the hydrolysis resistance and heat resistance of a polylactic acid resin are improved, the crystallization rate of polylactic acid is hardly improved, and thus productivity is still the problem. In addition, since the melting point of homopolylactic acid is around 170° C., the heat resistance is also the problem for practical uses.
As mentioned above, either the heat resistance or the crystallization properties can be improved by adding a phosphorous-based compound to a polylactic acid or a polylactic acid stereocomplex to obtain a polylactic acid resin composition. However, the polylactic acid resin composition satisfying both of the heat resistance and the crystallization properties has not been obtained.
It could therefore be helpful to provide a polylactic acid resin composition for forming a polylactic acid stereocomplex, which has excellent heat resistance, crystallization properties, mechanical properties, and durability, a molded product, and a method of manufacturing polylactic acid resin compositions. In particular, it could be helpful to provide a polylactic acid resin keeping excellent crystallization properties after deactivating a catalyst, and satisfying both of the heat resistance and the crystallization properties.